"It’s not a random pattern," says Aaron Lotz. "Out of all this data, that one factor—human life expectancy—was the determining factor for endangered and invasive birds and mammals." (Credit: Lwp Kommunikáció/Flickr)

Human life expectancy is rarely included in the ways people impact the environment, but new research finds it may be the key predictor in the rise of invasive and endangered birds and animals.

Scientists examined a combination of 15 social and ecological variables, including tourism, per capita gross domestic product, water stress, and political stability.

[related]

They then analyzed their correlations with invasive and endangered birds and mammals, which are two indicators of what conservationist Aldo Leopold termed “land sickness.”

“It’s not a random pattern,” says lead author Aaron Lotz, a postdoctoral scholar in the Department of Wildlife, Fish and Conservation Biology at the University of California, Davis when the study was conducted.

“Out of all this data, that one factor—human life expectancy—was the determining factor for endangered and invasive birds and mammals.”

Published in the journal Ecology and Society, the study analyzed data from 100 countries, which included roughly 87 percent of the world’s population, 43 percent of global GDP per capita, and covered 74 percent of the Earth’s total land area.

What the research shows:

New Zealand, the United States, and the Philippines have among the highest percentages of endangered and invasive birds.

New Zealand has the highest percentage of all endangered and invasive species combined, largely due to its lack of native terrestrial mammals. In the past 700 to 800 years since the country was colonized, it has experienced massive invasion by nonindigenous species, resulting in catastrophic biodiversity loss.

African countries have the lowest percentage of invasive and endangered birds and mammals. These countries have had very little international trade, which limits opportunities for biological invasion.

As GDP per capita—a standard measure of affluence—increases in a country, so does the percentage of invasive birds and mammals.

As total biodiversity and total land area increases in a country, so does the percentage of endangered birds. (Biodiversity in this context is not a measure of health but refers to the number of species in an area.)

The study indicates the need for a better scientific understanding of the complex interactions among humans and their environment, Lotz says.

“Some studies have this view that there’s wildlife and then there’s us. But we’re part of the ecosystem. We need to start relating humans to the environment in our research and not leave them out of the equation. We need to realize we have a direct link to nature.”

The study was co-authored by Craig Allen of the Nebraska Cooperative Fish and Wildlife Research Unit, which provided funding along with the James S. McDonnell Foundation-Studying Complex Systems.